The present invention relates to semiconductor design technology, and more particularly, to a semiconductor memory device using a magnetic tunnel junction element (MTJ).
In general, a dynamic random access memory (DRAM) device and a static random access memory (SRAM) device are volatile memory devices and thus have a disadvantage of losing data stored in memory cells when power to the device is turned off. Therefore, recently, researches for nonvolatile memory devices have been actively made. A magnetic random access memory (MRAM) device is a type of a magnetic memory device. In particular, the MRAM device has a nonvolatile characteristic and can achieve high integration. Furthermore, the MRAM device can perform a high-speed operation and has a low power consumption characteristic. Therefore, the MRAM has attracted considerable attention as a next generation semiconductor memory device.
A memory cell of the MRAM device includes one transistor for performing a switching operation in response to addresses provided from the outside and a magnetic tunnel junction element (MTJ) for storing information. The MTJ, which is a type of magnetic memory elements, has magneto-resistance (MR) varying according to a magnetization direction of two ferromagnetic substances. The MRAM device determines whether data stored in the MTJ is logical high state ‘1’ or logical low stage ‘0’ by detecting the variation of the magneto-resistance.
FIG. 1 illustrates a structure of a memory cell of a typical semiconductor memory device.
Referring to FIG. 1, the memory cell includes one transistor TR and one magnetic tunnel junction element MTJ.
The transistor TR performs a switching operation in response to addresses in an active operation. Thus, the transistor TR includes a source-drain path between a source line SL and the magnetic tunnel junction element MTJ, and a gate connected to a word line WL. As a result, the transistor TR is turned on/off according to whether the word line WL is activated or not.
The magnetic tunnel junction element MTJ includes a free layer 130A, a tunnel insulation layer 130B and a pinned layer 130C. Herein, the free layer 130A is formed of a ferromagnetic substance and its magnetization direction is changed by external impulse, e.g., a current applied to the magnetic tunnel junction element MTJ. The pinned layer 130C has a magnetization direction that is not changed by the external impulse. For illustration purposes, the magnetization direction of the pinned layer 130C is determined by a pinning layer (not shown) formed of an antiferromagnetic substance. The tunnel insulation layer 130B may be formed of a magnesium oxide (MgO) layer.
A tunnel current flows through the magnetic tunnel junction element MTJ according to voltages coupled to both ends of the magnetic tunnel junction element MTJ, and a magnetization direction of the free layer 130A is determined according to a direction of the tunnel current. In case where the magnetization direction of the free layer 130A is consistent with that of the pinned layer 130C, the resistance of the magnetic tunnel junction element MTJ is comparatively low. On the other hand, in case where the magnetization direction of the free layer 130A is not consistent with that of the pinned layer 130C, the resistance of the magnetic tunnel junction element MTJ is comparatively high (for example, to be higher than the comparatively low resistance). In general, a state that the magnetization direction of the free layer 130A is consistent with that of the pinned layer 130C corresponds to data ‘0’ and its opposite state corresponds to data ‘1’.
FIGS. 2A and 2B illustrate views for explaining a data writing operation of the magnetic tunnel junction element MTJ described in FIG. 1. FIG. 2A shows an operation of writing data ‘0’ to the magnetic tunnel junction element MTJ and FIG. 2B describes an operation of writing data ‘1’ to the magnetic tunnel junction element MTJ. For illustration purposes, it is assumed that the word line WL is activated. In this case, the magnetic tunnel junction element MTJ is included in a current path connecting the bit line BL and the source line SL.
First of all, the operation of writing the data ‘0’ to the magnetic tunnel junction element MTJ is described with reference to FIGS. 1 and 2A.
In the operation of writing the data ‘0’, a write driving circuit (not shown) drives the bit line BL with a write supply voltage and the source line SL with a ground voltage VSS. In other words, in the operation of writing the data ‘0’, a predetermined voltage greater than a certain level is supplied to the free layer 130A to be higher than that of the pinned layer 130C so that a current higher than a critical current is generated in a direction of the bit line BL->the magnetic tunnel junction element MTJ ->the source line SL. In this case, the magnetization direction of the free layer 130A is the same as that of the pinned layer 130C. That is, the resistance of the magnetic tunnel junction element MTJ is decreased and the operation of writing the data ‘0’ is completed.
Then, the operation of writing the data ‘1’ to the magnetic tunnel junction element MTJ is described with reference to FIGS. 1 and 2B.
In the operation of writing the data ‘1’, contrary to the operation of writing the data ‘0’, a predetermined voltage greater than a certain level is supplied to the pinned layer 130C to be higher than that of the free layer 130A so that a current higher than a critical current is generated in a direction of the source line SL->the magnetic tunnel junction element MTJ->the bit line BL. In this case, the magnetization direction of the free layer 130A is opposite to that of the pinned layer 130C. That is, the resistance of the magnetic tunnel junction element MTJ is comparatively high and the operation of writing the data ‘1’ is completed.
FIG. 3 illustrates a graph showing a tunnel magneto-resistance (TMR) characteristic according to temperature and voltage of the magnetic tunnel junction element MTJ illustrated in FIG. 1.
As can be seen from FIG. 3, the magnetic tunnel junction element MTJ has hysteresis and two stable states, i.e., a low resistance state and a high resistance state, according to the critical current flowing through the current path including the magnetic tunnel junction element MTJ and the direction of the critical current. The stable states are maintained although power to the device is turned off. Thus, the semiconductor memory device secures the nonvolatile characteristic of the data stored therein.
Meanwhile, a switching current of the magnetic tunnel junction element MTJ generally varies according to the temperature. Herein, the switching current means a current at a point where data ‘0’ or ‘1’ is written to the magnetic tunnel junction element MTJ. As can be seen from a region indicated by a dotted line in FIG. 3, the switching current of the magnetic tunnel junction element MTJ is comparatively low in the high temperature, e.g., 70° C, whereas it is comparatively high in the low temperature, e.g., 0° C. This characteristic of the magnetic tunnel junction element MTJ induces an unstable writing operation according to process, voltage and temperature (PVT).